Crystalline fused alumina and the manufacture thereof.



FRANEK 3'. TONE, OF NIAGARA FALLS, NEW ,YOBK, ASSIGNOR TO THE CABBORUNDUM COMPANY, OF NIAGARA FALLS, NEW YORK, A CORPOFATION OF PENNSYLVAN CBYSTALLINE FUSED ALUMINA AND THE MANUFACTURE THEREOF.

No Drawing.

. To all whom it may concern Be it known that I, FRANK J. TONE, acitizen of the United States, residing at Niagara Falls, in the county of Niagara and State of New York, have invented a new and useful Improvement in Crystalline Fused Specification of LetteIs'PatentQ Alumina and the Manufacture Thereof, of.

which the following isa full, clear, and exact description.

The object of this invention is the production of a new form of crystalline fused alumina for use as an abrasive material and possessing new and valuable characteristics.

Crystalline fused alumina is commonly produced by the fusion in the electric furnace of various aluminous materials orores,

' such as alumina, bauxite, emery or clay, re-

- impurities consisting mainly of iron oxid,

ducing at the same time a portion of the impurities or foreign material, when such are present in such ores, with the addition to the charge of a small percentage of carbon,;.the

silica and titanium oxid. The well known:

charge of the aluminous ore in a cruoi lei method of working consists in fusin a;

I trodes. The hearth or bottom of the furnace 1 consists of a refractory base generally of carbon and the sidesconsist of a'steel shell over the outer surface of which cooling water is caused to flow. This not only keeps the shell from being melted by the heat of the furnace, but maintains a layer ofv the unfused charge next to the shell.- Some cooling effect may also be produced on the fused alumina, but 'in view of the relatively greater amount of heat being liberated in the ,zone of fusion, this cooling effect is not I the furnace allowed to cool.

the furnace is removed after the ingot solidifies and it is then-broken down and crushed to the various sizes of grains required for grinding wheels and various abrasive purposes. This product, which is cooled in the furnace, I will hereafter refer to as regular product. t

. Crystalline'fused alumina made in accord ance with my improved method employs the The shell of shortest possible time.

same type of furnace and the same charge of raw materials. Instead, however, of building up a large ingot, the molten material afterbeingbrought to a high state of fluidity, is tapped out in small masses and quickly frozen. This freezing results in arresting the progress of crystallization and in the production of a crystalline structure of extreme fineness. Fused alumina is a material possessing extremely high crystallizing power. The crystal growth takes place with great rapidity and in order to arrest this growth and to produce alumina having the desired characteristics, I have found it' necessary to adopt means for rapidly removing the highly fused alumina from the furnace and freezing it in the A tapping hole is provided in the side of the furnace level ,with the hearth. The product in the fur- ,-nace is not only used, but is brought to a temperature several hundred degrees above .its melting point and at which it is of high fluidity. The tapping hole is then opened and the molten material allowed to flow out quickly into a mold. This mold is shallow and preferably large enough so that the thickness of the tapped material will not exceed eight inches, and preferably not over six inches. It may be madeof iron or steel and lined with carbon, or it may be made entirely ofsteel, thus giving a high freezing effect. The fused material solidifies almost instantly and after further cooling is removed frbm the mold, crushed and treated in the usual manner for the manufacture of abrasive grains. V

The regular product of crystalline fused alumina generally consists of from ninety to ninety-eight per cent. alumina, the remainder, which may be designated as im-, purities, being principally compounds of iron, silicon, titanium and lime. Micro scopic examination shows that this regular product consists of alumina crystals and a residual basis or material madeup principally of the impurities above mentioned contained in the ore. In general, the grains are anhedral to subhedral crystals and are roughly equidimensional, and there 1s. a

tendency toward a tabular habit. .The size Patented July 25, 1916.

Application filed December 1, 1914. Serial No. 875,009.

The residual basis is the foreign material separating to a considerable extent the A1 crystalsor found as inclusions therein. There is Without exception some foreign material in the A1 0 crystals. Its amount varies in different samples. It takes the form of fine. black dust-like particles commonly aggregated into small patches. Its character affects .the cohesive properties of the grains. The residual basis is usually amorphous though sometimes crystalline. It is opaque and partly-black. It. is not definitely determinable in character, but appears to'consist mainly of compounds of titanium, iron, and silica, though sometimes with a glass containing silica. As

, it has a different coeflicient of expansion from A1 0 its presence results in internal tension which, in turn, causes a weakness or brittleness'in the crystals. The residual l sis between the grains is thin, rarely measuring more than a few thousandths of a.

millimeter, but often broadens out in the larger. interstices between the crystals and penetrates them along crystallographical directions.

In addition to a glass associated with the black material and. really forming part of the residual basis, there is a variable amount of crystallized material in the form of small crystals having a strongly plate-like structure dueto a well developed cleavage. This l1es parallel to the boundaries or inthe large interstices sometimes penetrating into the Al O crystals. These crystals are usually very small, about a few thousandths to hundredths of a millimeter long and possess com monly a strong dichroism, sapphire blue to smoky gray.. The material appears to be.

unaxial and negative in optical character, and is believed to he also crystallized alumina. If this is A1 0 its color is doubtless due to dissolved impurities, which also seem to affect its mean index of refraction, rendering it a little different than that of'the alumina crystals of' the first generation.

Their cleavage causes a greater 'ease of j breakmg and they, together with the resid- 11211 basis, a-fl'ord lines or surfaces. of weakness which are important in determining the breaking down ofthema'ss as a whole. The size of the fragment which will. stand. up as an actual unit is thus seen to be in part a function of the size of the individual aluminacrystals of the first generation, slnce they are separated from each other in a large part by a more or less continual film of weaker cohesive power which furnishes a surface of fracture.

The finely crystallized product of fused crystalline alumina made by the present process from the same raw materials and of the same chemical composition as the regular product, examined with the eye alone, appears ,to be exceedingly dense and ahnost devoid of physical structure. The fracture is very tough and irregular. Under the microscope, either in thin sections or in crushed material, it is seen to consist in large part of an exceedingly fine grained, perfectly compacted and sealed aggregate of roughly equidimensional crystalline grains of A1 0 mingled, as hereinafter described, with minute particles of black, opaque substance consisting mainly of compounds of titanium, iron and silicon. The great majority of the A1 0 grains are anhedral crystals. Many are roundedor irregularjin outline and others are angular to sub-angular, while interspersed among others are crystals, flattened 0r plate-like in shape, showing parallel lines of incipient parting or cleavage, and less commonly, having' an almost fibrous appearance. Occasional grains may show "some trace of bound ing planes and may be termed subhedral crystals. e In size, the crystals of A1 0 will in general vary from those under .01 millimeter in diameter to, in exceptional cases, those measuring .3 millimeter in average dimension. The general average appears to. be a little over .1 millimeter. An actual measurement of several thousand grains gave an average dimension of .116 millimeter. In the-regular product of fused crystalline alumina, the grain while somewhat variable, will run on the average from three to six times as coarse, that is, of a different order of magnitude, using this term in its accepted petrographical significance.

In occasional samples, narrow streaks or hands of grains are found that will average somewhat finer than the rest and again there are bands and irregular patches where the crystals are largely of the short tabular form mingled with rounded and irregular grains, also bands or patches where small tabular and composite crystals occur. These bands and patches, although sometimes .well de-- eel fined as to outline, merge rapidly into the surrounding material. In this fine grained material are the usual minute opaque black particles noted in the regular product. This material forms minute specks, dusty patches,

larger irregular shaped patches, rounded globules and branching crystallizations. It is in general more evenly distributed throughout the mass asa whole than in the regular product of substantially'the same composition, and therefore, appears to be more abundant. No exact definition as to the degree of evenness or uniformity of distribution of the residual material can, in the nature of things, be given. As compared in this respect with the regular roduct, however,-the greater degree of uniiormity in the former is so marked'as to bereadily seen under. the microscope, and such examination alone will clearly distinguish the new prod- 'uct from the regular-product. This discementing the crystals of A1 0 The special features which characterize the finely crystallized product from the regular product of substantially the same chemical composition are: (1) the finer average grain; (2) the more general distribution of the opaque material through the mass, and

the relatively smaller size of thetabular (rarely blue) type of A1 0 crystal showing parting or cleavage lines.

The great fineness of the grain together with the irregular but tightly sealed contours of the individual crystals, as well as their vicarious orientations, make the fragments, such .as would be used for coarse numbers of abrasives or in making grinding wheels, more resistant to breaking forces, other things being equal, than the material made up of larger and consequently fewer crystals. in fragments of the new product, there are a great number of minute crystals standing atall angles to one another, and

since each crystal possesses its own particular direction of Variable resistance'to strain, it follows that such an aggregate, other things being equal, presents a greater resistance to breaking than an aggregate of larger crystals. Breaking strains impressed on one or more single small crystals extend be yond these grains into the fragment with less ease than they would in a fragment made up of few large crystals (or perhaps even of one large crystal) for in the latter case, the strains would be transmitted for a greater distance through crystallographically continuous material.

Owing to the relatively more general distribntion of opaque foreign material, as compared with the regular product, the seal of grain to grain is efiec'ted to a proportionally larger egitent by the substance of the A1 0 crystals themselves, resulting in a firmer union. The foreign material possesses a different coefficient of contraction from the A1 0 which alone is a source of weakness, particularly where the former through the mass, as a whole, and .diminishes the tendency to localized breaking. By the terms rapid and rapidly as There, is the same tend meter.

applied to the freezing operation herein, I refer to a freezing outside of the furnace, and under conditions in which the growth of crystals is completely checked within a small fraction of the time required to stop crystalline growth when the coolingoccurs within the furnace.

Iclaim: 1. As a new article of manufacture, crystalline fused alumina, in which the major portion of the alumina crystals have an average diameter of 2. As a new article of manufacture, crystalline fused alumina, in which the major portion of the aluminacrystals have a diameter of less than .15 millimeter.

3. As a new article of manufacture, crystallinc fused alumina, in which the major portion of the alumina crystals range in size from .01 to .3 millimeter.

4. As anew article of manufacture, crystalline fused alumina, in which the major portion of the alumina crystals are of an average lower order of magnitude than .3 millimeter.

5. {is a new article of manufacture, crystalline fused alumina comprising alumina crystals and residual basis and in which the residual basis is substantially uniformly distributed. 6. As a new article of manufacture, crystalline fused alumina, comprising. alumina ci'ystals and residual basis, and in which the major portion of the crystals have an average diameter of about .1 millimeter.

7. As a new article of manufacture, crystalline fused alumina, comprising alumina crystals and residual basis, and in which the major portion of the alumina crystals have a diameter-of less than .15 millimeter.

8. As a new article of manufacture, crystalline fused alumina, comprising alumina crystals and residual basis, and in which the major portion of the alumina crystals range in size from .01 to .3 millimeter.

9. As a new article of manufacture, crystalline fused alumina, comprising alumina crystals and residual basis, and in which the major portion of the alumina crystals are of an average lower order of magnitude than .3 millimeter.

" 10. As a new article of manufacture, crys talline fused alumina composed of alumina crystals and residual basis, the majornportion of the crystals being of a diameter of less than .15 millimeter and having vicarious orientations.

11. As a new article of manufacture, crystalline fused alumina composed of alumina crystals and residual basis, the major portion of the crystals having a diameter of less than .15 millimeter, and the residual basis being substantially evenly distributed.

12. As a new article of manufacture, crysabout one tenth milli less than .15 millimeter and in whichythe I talline fused alumina composed of alumina nace in a shallow mass, and then causing it to rapidly freeze.

18. The method of makingabrasive material which consists in fusing in an electric the sub furnace the substance or substances from \I Which the abrasive material is to be formed,

13. As a new article of manufacture, crysanifcooling the fused substance in a body of talline fused alumina composed of alumina sma l ratio of mass to superficial area. crystals and residual basis, the crystals be- 19. The method of making abrasive maing of an average order of magnitude less terial which consists in fusing aluminous than .3 millimeter, and the residual basis bematerial and cooling the fused material in ing substantially evenly distributed therein.

crystals and residual basis, themajor portion of the crystals having a diameter of seal of grain to grain is effected by stance of'the A1 0 crystals.

a body of small ratio of mass to superficial 14. The method of making crystalline area. fused alumina, which consists in fusing a 20. The method of making abrasive macharge of aluminous ore, heating it to a temterial which consists in fusing the substance perature sufliciently .boveits melting point or substances from which the abrasive mato bring it to'a state of high fluidity, reterial is to be formed and in cooling the moying it from the furnace, and then rapfused substance by exposing it in a thin idly freezing it. I layer to a cooling medium.

15. The method of making crystalline 21. The method of making abrasive mafused alumina, which consists in fusing a terial which consists in fusing aluminous charge of aluminous ore and carbon and material and in cooling the fused material heating it to atemperature sufficiently above by exposing it in a thin layer to a cooling its melting point to bring it to a state of medium; high fluidity, and tapping it out of the fur- 22. The method of making abrasive manace and then rapidly freezing it. terial which consists in fusing in an electric 16. The method of making crystalline furnace the substance or substances from fused alumina, which consists in fusing a which the abrasive material is to be formed, charge of aluminous ore in an electric furand cooling the entire body of the fused nace, heating it to a temperature sufliciently above its melting point to bring it to a'state to superficial area. of high fluidity, and then/ causing it to 23. The method of making abrasive mafreeze with sufficient rapidity to produce Al O crystals, the major portion of which material and cooling the entire body of the have an average diameter of less than .15 fused material in a body of small ratio of millimeter. mass to superficial'area.

17. The method of making crystalline In testimony whereof, fused alumina, which consists in fusing a my hand. charge of aluminous ore in an electric fur- FRANK J. TONE. nace, heating it to a temperature sufliciently above its melting point to bring it to a state of high fluidity, tapping it out of the fur- I have hereunto set Witnesses:

FRED I. PIERCE, C. S. DOUGLAS.

substance in a body of small ratio of 'mass terial which consists in fusing aluminous- 

